Power conversion systems are used to convert electrical power from one form to another. Grid tie converters may be used in a variety of situations in which power is converted from one form to provide power to a load and/or to a power grid. For example, grid tie converters are used to interface fuel cells, solar cells, wind energy systems, batteries and other energy storage systems with a power grid. These converters are often capable of regenerative operation, where grid power can be converted to DC power for charging batteries, or DC battery power can be converted to provide AC power to the grid. Islanding is a condition in which a portion of an area electrical power system or electric power supply (EPS) is energized solely by one or more local EPSs through associated points of common connection or points of common coupling (PCCs) while that portion of the area EPS is electrically separated from the rest of the area EPS. Islanding can be intentional or unintentional, and various standards including UL1741 and IEEE standards 929-2000 and 1547-2003 may require protection against continuous operation of an inverter or other converter and part of a utility load while isolated from them remainder of the electric utility system. In particular, continued power converter operation during unintended distributed resource islanding is generally undesirable since the utility cannot control the voltage and frequency in the island, and voltage or frequency excursions outside of acceptable ranges may damage or degrade customer equipment. Furthermore, islanding may create hazards for utility line workers by causing a line to remain energized when it is assumed to be disconnected from all energy sources. In addition, reconnection or re-closing into an island may result in line tripping or damage to distributed resource equipment or connected equipment due to out of phase closure. Accordingly various industry standards call for power converters and other distributed resources (e.g., synchronous machines, induction machines, power inverters/converters, etc.) that energize a portion of an area EPS through a PCC to detect unintentional islanding and cease to energize the area EPS.
Several different techniques have been developed for detection of unintentional islanding. However, conventional techniques have thus far been able to ensure accurate identification of unintentional islanding in grid tie converter applications, and hence a need remains for improved apparatus and methods for detecting islanding in grid tie converters.